1. Field of the Invention
The present invention relates to an image heating apparatus that can be suitably used as a heat fixing device equipped in a copying machine or a printer, and more particularly to an image heating apparatus provided with a heater having a heat generating resistor provided on a substrate and an elastic roller that forms, in cooperation with the heater, a nip portion through which a recording material that bears an image is conveyed.
2. Description of the Related Art
A film type fixing device has been practically used as a fixing device equipped in a copying machine or a printer. The film type fixing device has a heater made of a ceramic material, a fixing film made of polyimide or stainless steel etc. whose inner circumference is in contact with the heater and a pressure roller that forms a fixing nip portion in cooperation with the heater with the fixing film between.
In a type of film type fixing device, an elastic layer made of a silicone rubber or the like is provided on the fixing film. The elastic layer provided on the fixing film makes it possible to fix a toner image on a recording material in a surrounding manner. For this reason, this type of fixing device is mainly used in a full color printer.
FIG. 10 is a cross sectional view of a film type fixing device. The film type fixing device has a heater 1000 made of a ceramic material and a heater holder 1001 made of a heat resistant resin that holds the heater, both of which are provided in the interior of the fixing film 1002. The heater holder 1001 holds the heater 1000 all along the longitudinal direction of the heater holder 1001. The fixing film 1002 is opposed to the pressure roller 1003 so that a fixing nip portion N is formed between them. A thermosensitive element 1004 such as a thermal fuse or a thermistor like a thermostatic switch is provided on the heater 1000. A recording material P on which a toner image t has been formed is conveyed in the direction indicated by an arrow, and the toner image t on the recording material P is heated and fixed in the fixing nip portion N. Among the thermosensitive elements, the thermal fuse and the thermostatic switch serve as a safety device that operates upon sensing the heat, when the temperature of the heater 1000 rises abnormally due to, for example, malfunction of control circuit, to shut down power supply to the heater 1000. Among the thermosensitive element, the thermistor is adapted to detect the temperature of the heater 1000.
In designing the device, it is necessary to take into consideration delay in response of the thermosensitive element 1004 that may occur in the case where it cannot respond to rapid rise in the temperature of the heater 1000 when the temperature of the heater 1000 rises abnormally. When the abnormal heat generation by the heater continues due to delay in response of the thermosensitive element, the heater 1000 is likely to crack at the position at which the thermosensitive element 1004 is in contact with the heater 1000. The reason for this is as follows.
The heater holder 1001 is pressurized by the pressure roller 1003 from the heater side 1000 as shown in FIGS. 11A and 11B. The heater holder 1001 made of a heat-resistant resin has a hole 1001a into which the thermosensitive element for detecting the temperature is to be fitted, as shown in FIG. 11A. The rigidity of the heater holder is lower at the portion provided with the hole than the other portions of the heater holder. Accordingly, when the temperature of the heater 1000 rises abnormally, the portion provided with the hole is more likely to deform than the other portions of the heater holder, as shown in FIG. 11B. For this reason, a high stress acts on the portion of the heater 1000 that is adjacent to the portion of the heater holder provided with the hole, which leads to cracking of the heater 1000.
As a countermeasure against such heater cracking occurring at the position adjacent to the thermosensitive element 1004, a structure in which the hole into which the thermosensitive element is to be fitted is reinforced by a rib or the like has been proposed, as disclosed in Japanese Patent Application Laid-Open No. 2005-148460.
The structure disclosed in Japanese Patent Application Laid-open No. 2005-148460 is effective in reinforcing a heater holder to prevent bending of the heater itself in the case where the heater holder 1001 is prone to bend at the position of the hole upon abnormal temperature rise of the heater, namely in the case where the rigidity of the heater holder 1001 is low. Therefore, this structure is effective in preventing breakage of the heater 1000.
However, in the case where the rigidity of the heater holder is ensured to some extent, reinforcing the periphery of the hole into which the thermosensitive element is to be fitted is not sufficient in preventing cracking of the heater, in some cases.
This will be explained in the following with reference to FIG. 12A. As shown in FIGS. 12A and 12B, the heater 1001 is provided with a hole 1001a into which a thermosensitive element for detecting the temperature is to be fitted. When the temperature of the heater rises abnormally to reach the softening temperature of the heater holder, the seating portion of the heater holder that is in direct contact with the heater is softened. Since the heater 1000 is pressurized to the upward direction in FIGS. 12A and 12B by the pressure roller 1000, the heater sinks into the heater holder as shown in FIG. 12B when the heater seating surface of the heater holder is softened.
The temperature of the portion of the heater that is adjacent to the hole for the thermosensitive element rises exceedingly as compared to the other portions, since the heat in that portion is not taken away by the heater holder. Therefore, the portion of the seating surface of the heater holder that is adjacent to the hole for the thermosensitive element is likely to be softened. Accordingly, the portion of the heater that is adjacent to the hole for the thermosensitive element sinks into the heater holder by an amount larger than that in the other portions. Thus, the stress acting on the portion of the heater adjacent to the hole and its periphery becomes high, and there is a possibility that the heater may break.
As per the above, the conventional solution is not effective in the case where the phenomenon that the heater sinks into the heater holder due to softening of the heater seating surface of the heater holder occurs rather than bending of the heater holder.
Furthermore, a further increase in the speed of image forming apparatuses has been demanded in recent years. To increase the speed, it is necessary to give a larger quantity of heat to the recording material in a shorter time. This requires to supply a larger electric power to the heater to increase the overall quantity of heat generated.
When the power supplied to the heater becomes large, if the fixing device becomes uncontrollable for failure of a temperature control system or other reasons and a large amount of power is continuously supplied to the heater, high temperatures at which the seating surface of the heater holder easily melts are reached. Accordingly, the heater sinks into the heater holder, and the time until cracking of heater occurs is shortened. Therefore, cracking of the heater can occur, in some cases, before the thermosensitive element such as a thermostatic switch works.
When cracking of the heater occurs in this way, the heater cannot be used any longer, which is disadvantageous from the viewpoint of recycling of parts. In addition, there is the problem that a sufficient distance cannot be left between a portion to which the primary voltage is applied via a thermistor or the like provided on the heater and the secondary circuit or the ground portion. This sometimes leads to breakage of the secondary circuit, and an additional repair cost may be incurred.